Method and apparatus for tactile cueing of aircraft controls

ABSTRACT

A method and apparatus for tactile cueing of aircraft controls ( 21 ) is disclosed. The apparatus of the present invention warns pilots of approaching limits on certain aircraft performance parameters. The most common warnings are for rotor speed exceeding a moving limit. The present invention uses tactile cueing through the collective stick ( 21 ). Tactile cueing means that the pilot does not need to scan the intruments to ascertain proximity to the aforementioned limits. Instead, the pilot can operate the aircraft within proper limits by touch, while maintaining situational awareness outside of the cockpit ( 20 ). The method and apparatus of the present invention provides customary friction resistance up to a limit position that is continuously updated. According to the present invention, continued motion of the collective (21) in a direction beyond that limit position results in a breakout force and an increasing resistive force.

TECHNICAL FIELD

The present invention relates to aircraft control systems. Inparticular, the present invention relates to tactile cueing of aircraftcontrol systems.

DESCRIPTION OF THE PRIOR ART

Currently, the only method to monitor engine and rotor performance of ahelicopter or other rotorcraft in flight is visually, through theinformation displayed on various instruments, and/or audibly, with theuse of synthesized speech, recorded messages, tones, whistles, etc.These methods require the pilot to scan the instruments or expendcognitive power discerning the intent of the audible feedback. Duringhigh workload maneuvers, such as high speed turns and precision hovernear external hazards (buildings, vegetation, power lines, etc.), thepilot must maintain his gaze outside of the cockpit. Requiring the pilotto break that gaze to look at an instrument or caution light creates ahigh risk. Parameter condition is achieved at the expense of aircraftsituational awareness. One problem with these monitoring methods is thatthe pilot must stay informed of certain operating parameters, becauseexceeding the operational limits of these parameters can lead tosignificant degradation of aircraft performance and/or failure of vitalaircraft components.

Over the years, many different methods have been developed to addressthis problem. These methods can be categorized broadly into threeclasses. In the first class, which is most often implemented withfly-by-wire systems, the pilot's control inputs are electronicallyinterrupted, and only as much of the pilot's inputs as are allowed arepassed to the control system, so that the aircraft will not exceed anylimits when responding. With the methods of this class, the pilotretains full control motion, but his authority is usurped. Therefore,this method does not really address the problem of keeping the pilotinformed of proximity to a limit. Instead, it imposes a rigid set ofrules that describe the flight envelope.

In the second class of monitoring methods, a tactile cue that retardsthe motion of the control, referred to as a “hard stop,” is provided.Such a hard stop is almost universally rejected.

In the third class of methods, a tactile cue that shakes the control isprovided. In these methods, the tactile cue is only a classifier, i.e.,either the pilot is violating a limit, or he is not. There is no“leading” information or forewarning. These methods are typically themost easy to implement, but they do not provide any informationregarding the degree of limit exceedance.

SUMMARY OF THE INVENTION

There is a need for a tactile cueing system for an aircraft controlsystem that provides limit proximity information to the pilot on acontinuous basis, without diverting the pilot's attention from theprimary task of flying the aircraft, without interfering with thepilot's control motion, and without artificially changing thesensitivity of the aircraft to the pilot's control inputs.

Therefore, it is an object of the present invention to provide a tactilecueing system for an aircraft control system that provides limitproximity information to the pilot on a continuous basis, withoutdiverting the pilot's attention from the primary task of flying theaircraft, without interfering with the pilot's control motion, andwithout artificially changing the sensitivity of the aircraft to thepilot's control inputs.

It is another object of the present invention to provide a tactilecueing system for an aircraft control system that can be mechanicallyand electrically retrofitted to existing aircraft.

It is yet another object of the present invention to provide a tactilecueing system for an aircraft control system that utilizes existingprocessing and aircraft data resources commonly utilized in Flight DataRecorder (FDR) systems and Health and Usage Monitoring Systems (HUMS).

It is yet another object of the present invention to provide a tactilecueing system for an aircraft control system that allows“eyes-out-the-cockpit” operation during demanding maneuvers utilizingthe full torque envelope of an aircraft.

These objects are achieved by providing a simple and cost effectivemechanical spring and electric motor system that generates the desiredtactile force cueing to the aircraft control system. The method andapparatus for tactile cueing of aircraft controls according to thepresent invention comprises a parameter prediction and a “soft-stop”tactile cue.

The parameter prediction uses a computer, associated software, andsensors of control position, engine parameters, and rotor performance topredict a future value of certain parameters based upon current values.Any number of algorithms can be applied to the prediction problem,including, but not limited to, Kalman filtering, extended Kalmanfiltering, linear prediction, trending, multi-variable surface fits ofmeasured data, simple analytical expressions, artificial neuralnetworks, and fuzzy logic. Some of the sensors measure current values ofair data, such as airspeed and rate of descent. Other sensors measureperformance parameters, such as engine torque, exhaust gas temperature,and rotor speed. Still other sensors measure pilot inputs throughcontrol displacement and rate information. All of this sensed data issent to the aircraft's flight control computers to prepare the data foranalysis.

Based on the selected algorithm, the parameter prediction is made of afuture value of the desired performance parameters. This predicted valueis then passed to a soft-stop cueing algorithm. The soft-stop algorithmis a “floating ground” algorithm that does not require additional sensedpositions of either side of the spring cartridge. This reduces the costof the system and increases reliability by reducing complexity. The useof a stepper motor combines braking capability and precise positioncontrol of the “floating ground” side of the spring cartridge withoutthe requirement of additional sensors. Although the present invention isdescribed below with respect to engine torque management, it will beappreciated that the method and apparatus of the present invention maybe used to manage other aircraft parameters, such as rotor speed andengine temperature, or any other aircraft operational parameter thatrequires limiting and/or reducing control inputs.

The soft-stop tactile cue is achieved by use of a force gradient springcartridge placed in parallel with an existing control linkage. One endof the spring cartridge is attached to the existing control linkage, andthe other end of the spring cartridge is attached to an actuator arm ofa stepper motor. A microswitch is placed in-line with the springcartridge to prevent inadvertent stick motion when the predicted torquedrops below the limit torque and the stepper motor is ready to return toa free-wheeling mode. Furthermore, a stick shaker can be attached to thecollective stick to provide an additional tactile cue.

As the pilot operates the collective stick, the existing control linkagedrives one end of the spring cartridge. When the aircraft is beingoperated within its envelope limits, the stepper motor shaft is free tomove in either direction as dictated by forces applied to the actuatorarm. The forces applied to the actuator arm are those transmitted by thespring cartridge and are due to the motion of the collective. Duringsuch time, the actual and predicted values of torque are below thetorque limit. However, if the maximum of either the predicted or actualtorque exceeds the limit, the software directs these activities.

First, an engage flag for the stepper motor is set true, making thestepper motor act like a magnetic brake. Thus, if the pilot continuespulling up on the collective, the microswitch shows its true stateindicating that the spring cartridge is in tension. The spring cartridgethen supplies a resistive force consisting of a breakout force and anincreasing force proportional to the amount of exceedance. Once thepilot pushes down on the collective releasing the spring tension, themicroswitch changes to its false state causing the stepper motor torevert to free-wheeling mode, thereby removing any resistance tocorrective action. When the engage flag changes to true, the currentlocation of the collective is recorded and serves as an initial valuefor both the actual location and the commanded location of thecollective stick.

Second, a collective limit position (CLIP) is calculated. Thiscalculation determines where the collective should be so that the torquewill just equal the limit at the future time, referred to as theprediction horizon. The CLIP is measured relative to the currentlocation of the collective position, so only a change or delta needs tobe calculated. The calculation itself comes from the amount the torqueexceeds the limit multiplied by the gain relating inches of collectivestick to change in torque. The CLIP is then added to the commandedlocation for the collective step.

Third, a step command is issued to the stepper motor. If the commandedlocation is below the actual location, a “down” step is issued. If thecommanded location is above the actual location, an “up” step is issued.Coincident commanded and actual location issues a “zero” step. Thestepper motor then moves one end of the spring cartridge accordingly. Ifthe pilot maintains just the breakout force on the collective, thestepper motor actually drives the pilot's hand to track exactly thetorque limit. If the pilot maintains the collective in one position, hefeels the force modulate according to the degree of exceedance.

Fourth, if the exceedance is greater than a selected additionalincrement above the limit, the stick shaker is activated.

These four evaluations are repeated every computational frame. The exactlogic for stepper motor engagement and direction involves a truth tablethat uses values of torque exceedance, current and previous steppermotor engagement, and state of the microswitch.

An important aspect of the present invention is the fact that thecorrective action by the pilot for torque exceedance, rotor droop, andexhaust gas temperature is to push the collective down. In order to cueagainst a limit exceedance on all of these parameters, the system needonly determine if any exceedance exists individually. If so, the systemstarts the cueing process, then calculates the CLIP for each parameterthat is exceeding its limit, and uses the most conservative answer.

Finally, the limits are not constants, but are instead functions ofairspeed and other parameters. For instance, the torque limit changes instep fashion at a certain speed, for example V_(q). In order to preventa sudden change in cueing force due to a sudden change in limit value,the limit value is slowly changed as a function of airspeed proximity toV_(q), and the rate at which the airspeed approaches V_(q).

The present invention provides many significant benefits and advantages,including: (1) the use of electro-hydraulic actuators for inducingcontrol force feel is avoided, resulting in less complexity, morereliability, and lower costs for maintenance and repair; (2) the tactilecueing stimulates a sense that is not already saturated, therebyrequiring significantly reduced cognitive effort; (3) an algorithm thatcontinuously updates the limit position of all parameters over which thecollective has significant influence is used; (4) pilot intent is notinterfered with, so that if a pilot wants to pull through the cue, thissystem will resist, but not stop that action; (5) the system employs acrisp, unambiguous cue with an optional shaking cue, as opposed to ashaking cue alone; (6) the crisp tactile cues permit more accuratetracking of the limit than do shaking cues; (7) inadvertent over-torqueevents can be eliminated, while reducing pilot workload; (8) helicopteroperational safety is improved by reducing pilot workload associatedwith avoiding certain operational parameter exceedances during demandingmaneuvers; (9) more than one limit can be cued by the collective stick,i.e., rotor speed, engine torque, and exhaust gas temperature; (10)different limits can be fused into one conservative limit that istransmitted to the cueing force controller; (11)“leading” feedback,i.e., the cue the pilot feels is one calculated to prevent a limitexceedance at some future time, usually about a half second in thefuture, can be provided; (12) existing flight control linkages areretained, as the cueing algorithm uses a minimum of system sensors, andsolves the problem of limited bandwidth of the cueing motor; (13) thesystem can be applied to other aircraft controls, such as the cyclic andpedals; and (14) the system is easily retrofitted to a wide range ofexisting aircraft.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an aircraft having a tactile cueingsystem according to the present invention.

FIG. 2 is a simplified schematic of the tactile cueing system foraircraft controls according to the present invention.

FIG. 3 is an exemplary configuration of the simplified representation ofthe tactile cueing system according to the present invention.

FIG. 4 is a detailed schematic of the tactile cueing system according tothe present invention.

FIG. 5 is a table of flight data parameters used by the tactile cueingsystem according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The method and apparatus of the present invention uses tactile feedbackto cue a pilot of impending exceedance of one or more operationalparameters of an aircraft. The present invention enables the pilot tomaintain “eyes-out-the-window” references during high-workloadmaneuvering tasks. Although the present invention is described withregard to a helicopter and HUMS parameters, it should be understood thatthe present invention is not limited to such applications, but may beused as an independent system on any rotorcraft or other aircraft, withor without a HUMS.

As with any helicopter limit cueing system, the cueing required forclosed-loop torque management must be timely and unambiguous. Simplyintroducing a soft-stop at the static collective position where anexceedance is first expected to occur is insufficient due to the falserelief cues that may result. For example, if the collective is loweredto relieve the force cue, the aircraft could still be in an exceedancecondition due to the application of other control inputs. In othersituations, the cueing must be able to adapt to airspeed dependentlimits on torque. Additional requirements for helicopter limit cueingsystems flow down from safety, certification, performance, and cost andweight considerations, as follows: (1) the soft-stop must not have afailure mode that a pilot cannot overcome with tolerable control forces;(2) the prediction algorithm must provide a suitable lead-time; (3)discontinuous torque limits must not cause discontinuous cueing forces;(4) system costs, including the cost of retrofitting existing aircraft,must be kept to a minimum; and (5) system reliability must be high.

Although the present invention is described below with respect to enginetorque management, it will be appreciated that the method and apparatusof the present invention may be used to manage other aircraftparameters, such as rotor speed and engine temperature, or any otheraircraft operational parameter that requires limiting and/or reducingcontrol inputs.

Referring to FIG. 1 in the drawings, an aircraft 10 having a tactilecueing system 11 according to the present invention is illustrated.Although aircraft 10 is shown as a helicopter, it will be appreciatedthat aircraft 10 may be a fixed wing aircraft, a tilt rotor aircraft, orany other rotorcraft, such as a tilt wing aircraft or a tail sitteraircraft. Aircraft 10 includes a fuselage 12, a drive means 18, and amain rotor 14. Torque imparted to fuselage 12 by main rotor 14 iscounter-acted by a tail rotor assembly 16 mounted on a tail portion 22of fuselage 12. Main rotor 14 and tail rotor assembly 16 are powered bydrive means 18 under the control of a pilot in a cockpit 20.

Referring now to FIG. 2 in the drawings, the preferred embodiment oftactile cueing system 11 is illustrated in a simplified schematic.Tactile cueing system 11 includes a force gradient spring cartridge 13placed in parallel with an existing control linkage 15. One end ofspring cartridge 13 is coupled to existing control linkage 15, and theother end of spring cartridge 13 is coupled to an actuator arm 17 of anelectric stepper motor 19. Control linkage 15 is coupled to a collective21 via a mixing lever 23. A switching means, or microswitch 25, isoperably associated with spring cartridge 13, preferably by beingdisposed in-line with spring cartridge 13, to prevent inadvertent motionof collective 21 when the predicted torque drops below the limit torqueand stepper motor 19 is ready to return to a free-wheeling mode. Aposition transducer 27 is operably associated with control linkage 15 toprovide position data for control linkage 15. In addition, a stickshaker 29 may be optionally attached to collective 21 to provide anadditional tactile cue. As is shown, stepper motor 19, microswitch 25,position transducer 27, and stick shaker 29 are all coupled to a systemcomputer 31.

Referring now to FIG. 3 in the drawings, one exemplary configuration ofthe simplified representation of tactile cueing system 11 of FIG. 2 isillustrated. In the preferred embodiment, spring cartridge 13, controllinkage 15, stepper motor 19, mixing lever 23, and microswitch 25 aredisposed beneath the cabin floor of aircraft 10.

Referring now to FIG. 4 in the drawings, tactile cueing system 11 isshown in a more detailed schematic. FIG. 4 illustrates theinter-relation of tactile cueing system 11 to other control systems ofaircraft 10. Tactile cueing system 11 is controlled by a collectivecueing processor (CCP) 51 that is powered by aircraft 10. If aircraft 10includes a HUMS, it is preferred that the central processing unit (CPU)from the HUMS be used to perform the processing functions of CCP 51.This is the configuration illustrated in FIG. 4. In such applications,CCP 51 is preferably based on the HUMS Processor Module (HPM) availablefrom Smiths Aerospace Electronic Systems. If aircraft 10 does notinclude a HUMS, then CCP 51 may comprise a stand alone unit. Inaddition, the processing functions of CCP 51 may be performed by aflight control computer 59, provided aircraft 10 includes such a flightcontrol computer 59, and that flight control computer 59 has sufficientcomputing capacity to perform the processing functions of CCP 51. Ofcourse, it will be appreciated that CCP 51 may also be a stand aloneunit in applications in which aircraft 10 includes a HUMS. In thepreferred embodiment, an additional interface card 55 to drive thehigh-current cueing devices, such as stepper motor 19, stick shaker 29,and warning lights 53, is integrated into tactile cueing system 11.

The HPM is preferably a 603 e PowerPC processor based system with serialand discrete input/output capability. As well as having specializedavionics interface devices, the HPM is also fitted with four universalasynchronous receiver/transmitter (UART) serial interfaces 57, overwhich the HPM receives data from aircraft flight control computers 59.

Interface card 55 is used to enable CCP 51 to generate discrete outputsignals to drive the cueing devices. Interface card 55 inverts thesignals to ensure that if power is removed from CCP 51, stepper motor 19is allowed to free wheel, stick shaker 29 and the over-torque indicatorare disabled, and failure warning indicator 53 is illuminated.

CCP 51 uses flight data information from a data acquisition system ofaircraft 10 to identify the aircraft flight condition and predict thetorque level. When the torque is predicted to exceed the transmissionlimit, a cue is provided. The cue can be generated in a number of forms,including collective force cueing, a stick shaker, voice warning, orvisual warning.

As the pilot operates collective 21, control linkage 15 drives one endof spring cartridge 13. When aircraft 10 is being operated within itsenvelope limits, the shaft of stepper motor 19 is free to move in eitherdirection as dictated by forces applied to actuator arm 17. The forcesapplied to actuator arm 17 are those transmitted by spring cartridge 13and are due to the motion of collective 21. During such time, the actualand predicted values of engine torque are below the torque limit.However, if the maximum of either the predicted or actual engine torqueexceeds a selected limit, the system computer 31 directs theseactivities.

First, an engage flag for stepper motor 19 is set true, making steppermotor 19 act like a magnetic brake. Thus, if the pilot continues pullingup on collective 21, microswitch 25 shows its true state indicating thatspring cartridge 13 is in tension. Spring cartridge 13 then supplies aresistive force consisting of a breakout force and an increasing forceproportional to the amount of exceedance. Once the pilot pushes down oncollective 21 releasing the spring tension, microswitch 25 changes toits false state causing stepper motor 19 to revert to free-wheelingmode, thereby removing any resistance to corrective action. When theengage flag changes to true, the current location of collective 21 isrecorded and serves as an initial value for both the actual location andthe commanded location of collective 21.

Second, a collective limit position (CLIP) is calculated. Thiscalculation determines where collective 21 should be so that the torquewill just equal the limit at the future time, referred to as theprediction horizon. The CLIP is measured relative to the currentlocation of the collective position, so only a change or delta needs tobe calculated. The calculation itself comes from the amount the torqueexceeds the limit multiplied by the gain relating inches of collectivestick to change in torque. The CLIP is then added to the commandedlocation for the collective step.

Third, a step command is issued to stepper motor 19. If the commandedlocation is below the actual location, a “down” step is issued. If thecommanded location is above the actual location, an “up” step is issued.Coincident commanded and actual location issues a “zero” step. Steppermotor 19 then moves one end of spring cartridge 13 accordingly. If thepilot maintains just the breakout force on collective 21, stepper motor19 actually drives the pilot's hand to track exactly the torque limit.If the pilot maintains collective 21 in one position, he feels the forcemodulate according to the degree of exceedance.

Stepper motor 19, coupled with the spring cartridge 13, applies therequired cueing force. In normal operations, below the torque limit,stepper motor 19 is designed to free wheel and spring cartridge 13 doesnot apply force to collective 21. If a torque exceedance is predicted,stepper motor 19 is engaged and an immediate collective force cue istransmitted to the pilot. The force cue preferably consists of an8-pound breakout force at the torque limit plus a 1.4 pound per inchforce gradient. Because collective position for limit torque will varywith flight condition and maneuver requirements, the resulting positionis a dynamic value that requires constant update.

Fourth, if the exceedance is greater than a selected additionalincrement above the limit, stick shaker 29 is activated.

These four evaluations are repeated every computational frame. The exactlogic for stepper motor engagement and direction involves a truth tablethat uses values of torque exceedance, current and previous steppermotor engagement, and state of the microswitch.

As set forth above, an important aspect of the present invention is thefact that the corrective action by the pilot for torque exceedance,rotor droop, and exhaust gas temperature is to push collective 21 down.In order to cue against a limit exceedance on all of these parameters,the system need only determine if any exceedance exists individually. Ifso, tactile cueing system 11 starts the cueing process, then calculatesthe CLIP for each parameter that is exceeding its limit, and uses themost conservative answer.

Finally, the limits are not constants, but are instead functions ofairspeed and other parameters. For instance, the torque limit changes instep fashion at a certain speed, for example V_(q). In order to preventa sudden change in cueing force due to a sudden change in limit value,the limit value is slowly changed as a function of airspeed proximity toV_(q), and the rate at which the airspeed approaches V_(q).

Flight control computers 59 provide flight data to control softwareresiding on CCP 51, which sends applicable tactile cues to the pilot.The control software uses current control positions and aircraft flightparameters from flight control computers 59 to perform a neural networkbased prediction of future mast torque. A prediction using thecollective rate is also possible to compensate for aggressive collectiveinputs. CCP 51 controls the engagement and position of stepper motor 19.

In the preferred embodiment, tactile cueing system 11 uses flight dataavailable from a typical HUMS system to provide the required input fortactile cueing. A major cost driver for a typical FDR or HUMSinstallation involves the acquisition of flight data from thepredominately analogue transducers found on civil rotorcraft, and theprocessors required to implement HUMS applications. This means that theaddition of tactile cueing system 11 on an aircraft already equippedwith HUMS can be achieved at minimum additional cost.

Referring now to FIG. 5 in the drawings, a table of flight dataparameters is illustrated. Three separate polynomial neural networks(PNN) predict the torque simultaneously. These predictions are comparedto the current torque, and a final weighted average for future torque isproduced. The preferred PNNs were developed using the group method ofdata handling (GMDH) algorithm. A major feature of the GMDH algorithm isthat it produces deterministic algebraic expressions suitable formeeting software certification requirements. Each PNN uses anindependent set of flight data parameters from aircraft 10. Theparameters are preferably grouped into the following categories:airframe, engine and pilot. The algorithm package has been written suchthat a different set of PNNs can be used depending on the currentaircraft flight condition. Two exemplary flight conditions are: (1)above 40 knots; and (2) below 40 knots.

As set forth above, tactile cueing system 11 comprises a parameterprediction and a “soft-stop” tactile cue. The parameter prediction usesa computer, associated software, and sensors of control position, engineparameters, and rotor performance to predict a future value of certainparameters based upon current values. Any number of algorithms can beapplied to the prediction problem, including, but not limited to, Kalmanfiltering, extended Kalman filtering, linear prediction, trending,multi-variable surface fits of measured data, simple analyticalexpressions, artificial neural networks, and fuzzy logic. Some of thesensors measure current values of air data, such as airspeed and rate ofdescent. Other sensors measure performance parameters, such as enginetorque, exhaust gas temperature, and rotor speed. Still other sensorsmeasure pilot inputs through control displacement and rate information.All of this sensed data is sent to the aircraft's flight controlcomputers to prepare the data for analysis.

Based on the selected algorithm, the parameter prediction is made of afuture value of the desired performance parameters. This predicted valueis then passed to a soft-stop cueing algorithm. The soft-stop algorithmis a “floating ground” algorithm. This means that a fixed referencepoint for the position of spring cartridge 13 is not necessary. Byutilizing this floating ground algorithm, additional sensors to detectthe positions of either side of spring cartridge 13 are not necessary.This reduces the cost of the system and increases reliability byreducing complexity. The use of stepper motor 19 combines brakingcapability and precise position control of the floating ground side ofspring cartridge 13 without the requirement of additional sensors.

The cueing algorithm functions as an inverse model. The maximum of thepredicted torque and the measured torque is known as the test torque.The test torque is compared to the torque limit, which varies withflight condition. If the test torque rises above the torque limit, themotor engages, establishing the ground for spring cartridge 13. Thepilot will feel the breakout force, and an increasing gradient forcewith continued upward movement of collective 21. In flight,determination of the test torque value is an ongoing process, andcommands to actuate stepper motor 19 are continuously computed in orderto drive the cue to correspond with the limit collective position. Ifthe pilot lowers collective 21, decreasing the torque, stepper motor 19is disengaged and becomes freewheeling. The inertia of stepper motor 19is small enough that no appreciable inertial resistance to collectivemotion is detected. The control algorithm also adapts to discontinuoustorque limits within the helicopter flight envelope, using a ramp thatis a function of the proximity and rate of approach to thediscontinuity.

Tactile cueing system 11 results in significant advantages in terms ofsystem airworthiness considerations. During normal operation tactilecueing system 11 is transparent to the pilot. In the event of animpending torque exceedance, the pilot can still apply any requiredcollective input by pulling through the breakout and gradient force.This is a very intuitive reaction. The use of stepper motor 19 makes thepossibility of an actuator hard-over very improbable. In the event of amechanical jam the pilot can still fly through spring cartridge 13without objectionable collective forces.

It is apparent that an invention with significant advantages has beendescribed and illustrated. Although the present invention is shown in alimited number of forms, it is not limited to just these forms, but isamenable to various changes and modifications without departing from thespirit thereof.

1. A tactile cueing system for an aircraft having a control mechanismcomprising: an electronic stepper motor; a force gradient spring coupledat one end to the stepper motor and at the other end to the controlmechanism; and a cueing processor coupled to the stepper motor forcontrolling the stepper motor; wherein the stepper motor and the springimpart a tactile cue to the control mechanism if the control mechanismis moved into a position that would cause the aircraft to exceed anoperational limit of at least one selected operational parameter.
 2. Thetactile cueing system according to claim 1, wherein the tactile cue is aresistive breakout force.
 3. The tactile cueing system according toclaim 1, wherein the stepper motor and spring also impart a secondtactile cue that is in proportion to the amount of exceedance.
 4. Thetactile cueing system according to claim 3, wherein the second tactilecue is an increasing force.
 5. The tactile cueing system according toclaim 1, wherein the tactile cue modulates according to the degree ofexceedance.
 6. The tactile cueing system according to claim 1, furthercomprising: a shaker mechanism coupled to the control mechanism; whereinthe shaker mechanism imparts a third tactile cue to the controlmechanism.
 7. The tactile cueing system according to claim 6, whereinthe shaker mechanism imparts the third tactile cue to the controlmechanism only when the amount of exceedance is greater than apredetermined amount over the operational limit of any one of theoperational parameters.
 8. The tactile cueing system according to claim1, further comprising: a means for sensing the actual values of eachselected operational parameter of the aircraft; and a parameterprediction algorithm programmed into the cueing processor for predictingfuture values of each selected operational parameter; a cueing algorithmprogrammed into the cueing processor for comparing the actual values andthe predicted future values of the operational parameters; wherein thetactile cue is generated by the stepper motor and the spring based inresponse to an output from the cueing algorithm.
 9. The tactile cueingsystem according to claim 8, wherein the cueing algorithm is a floatingground algorithm for which the actual absolute sensed position of eitherend of the spring is not required.
 10. The tactile cueing systemaccording to claim 8, wherein the parameter prediction algorithmcalculates a future position of the control mechanism that correlates toa position that would cause the aircraft to exceed the operational limitof any one of the selected operational parameters, the future positionbeing measured relative to the actual position of the control mechanism.11. The tactile cueing system according to claim 8, further comprising:a switching means operably associated with the spring for preventinginadvertent movement of the control mechanism when the predicted futurevalues of the selected operational parameters do not exceed the selectedoperational limits of the aircraft.
 12. The tactile cueing systemaccording to claim 8, wherein the parameter prediction algorithmincludes at least one technique for estimating the future values of theoperational parameters.
 13. The tactile cueing system according to claim8, wherein the parameter prediction algorithm and the cueing algorithmrun continuously and periodically update during operation of theaircraft.
 14. The tactile cueing system according to claim 1, wherein asingle corrective movement of the control mechanism prevents theaircraft from exceeding the operational limits of all of the selectedoperational parameters.
 15. The tactile cueing system according to claim1, wherein the selected operational parameters include engine torque,rotor droop, and exhaust gas temperature.
 16. The tactile cueing systemaccording to claim 1, wherein the operational limits are functions ofthe selected operational parameters, other operational parameters, or acombination of some or all of the selected operational parameters andthe other operational parameters.
 17. The tactile cueing systemaccording to claim 1, further comprising: a position transducer operablyassociated with the control mechanism for providing data on the positionof the control mechanism.
 18. The tactile cueing system according toclaim 1, further comprising: a health and usage monitoring system;wherein the health and usage monitoring system is coupled to the cueingprocessor.
 19. The tactile cueing system according to claim 18, whereinthe health and usage monitoring system performs the processing functionsof the cueing processor.
 20. The tactile cueing system according toclaim 1, further comprising: a flight control computer for controllingthe aircraft; wherein the flight control computer is coupled to thecueing processor.
 21. The tactile cueing system according to claim 20,wherein the flight control computer performs the processing functions ofthe cueing processor.
 22. The tactile cueing system according to claim1, wherein the stepper motor moves freely while the control mechanism isoperated within the operational limits of the aircraft.
 23. An aircraftcomprising: a fuselage; a drive means carried by the fuselage; a controlmechanism for controlling the aircraft; and a tactile cueing systemcoupled to the control mechanism comprising: an electronic steppermotor; a force gradient spring coupled at one end to the stepper motorand at the other end to the control mechanism; and a cueing processorcoupled to the stepper motor for controlling the stepper motor; whereinthe stepper motor and the spring impart a tactile cue to the controlmechanism if the control mechanism is moved into a position that wouldcause the aircraft to exceed an operational limit of at least oneselected operational parameter.
 24. The aircraft according to claim 23,further comprising: a means for sensing the actual values of eachselected operational parameter of the aircraft; and a means forpredicting future values of each selected operational parameter; whereinthe tactile cue is generated by the stepper motor and the spring basedupon a comparison of the actual values and the predicted future valuesof the operational parameters.